Spider monkey society is sexually segregated

Ew, boys! A species of spider monkey has been found to live in strictly sexually segregated societies, apparently because the males attack the females if they spend too much time together. They are the first non-human primate species known to systematically separate along gender lines.

Geoffroy's spider monkeys, Ateles geoffroyi, live in loose groups of a few dozen individuals, and anecdotal evidence suggested they are sexually segregated. To find out if that was true, Kayla Hartwell of the University of Calgary, Canada, and colleagues spent almost two years studying a group of 34 monkeys in the rainforests of Belize.

They tracked the monkeys' movements and found that males and females lived separately for 15 out of 23 months. While males foraged in groups, females often foraged with just their infants. Only when food was scarce did males and females come together.

"Spider monkey sexual segregation arises from differences between male and female behaviour," says Hartwell. "Males and females have evolved to be so different that spending time together quickly becomes hostile."

Males are friendly to each other, spending hours mutually grooming and falling asleep hugging. But they are aggressive towards females and attempt to dominate them.

Hartwell suggests that males need to expend a lot of energy patrolling their territory and driving off rival males. To keep up their strength, they eat more ripe fruits than females, often chasing the females away from fruiting trees. The females mostly eat less nutritious leaves and travel far less, spending most of their time tending their infants. They may accept the situation because they need the males' protection. "If they are going to have to eat leaves they might as well do so segregated and free of aggression," says Hartwell.

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